1. Field of the Invention
The present invention generally relates to methods for dewatering sludge and, in particular, alum sludge produced at a water treatment plant. Experiments have been conducted which show that treating alum sludge with hydrogen peroxide (H.sub.2 O.sub.2) prior to standard dewatering processes increases the efficiency of those processes.
2. Description of the Prior Art
The use of aluminum sulfate (alum or filter alum) as a coagulant for treating surface water was introduced in 1884. When this salt is added to water, the aluminum ion hydrolyzes by reactions that consume alkalinity in water. Equation 1 is representative of the reaction in water: EQU Al(H.sub.2 O).sup.3+.sub.6 +.sup.-3 HCO.sub.3 .fwdarw.Al(OH).sub.3 .multidot.3H.sub.2 O(S)+3CO.sub.2 +3H.sub.2 O 1.
The above reaction produces a gelatinous hydroxide which carries suspended material with it as it settles. In addition, it is likely that positively charged hydroxyl bridged dimers and higher polymers are formed which interact specifically with colloidal particles to bring about coagulation. These gelatinous flocs are heavier than water and settle by gravity. The disposal of these sediments (sludges) from water treatment plants is not a new problem. The majority of water treatment plants have disposed of these sludges by discharging them to a downstream point of the raw water source. Under current federal and state laws, these sludges are considered pollutants and certain requirements must be met for their proper disposal.
Water plant sludges are often characterized by high water content (generally, low suspended solids concentrations), high resistance to mechanical or gravity dewatering, and having other problems associated with their handling and ultimate disposal. These sludges are highly variable in composition and contain both the concentrated materials removed from raw water and the chemicals added in the treatment processes. Alum sludge dewatering remains one of the most difficult and elusive of the environmental engineering challenges. One of the most troubling problems with alum sludge dewatering is that there seems to be no accepted means to evaluate the ease with which a particular sludge will release its water.
U.S. Pat. No. 3,720,608 to Stauffer discloses a method and apparatus for conditioning and disposing of alum sludge from a water treatment plant. The method requires subjecting the sludge to boiling temperatures (212.degree. F.) for thirty minutes, then separating the solids from the liquid fraction by decanting to produce a low moisture content sludge cake which is suitable for disposal. The time and cost of energy for heating the alum sludge which are specified by Stauffer are not suitable for modern requirements. What is needed is a low cost treatment scheme which can rapidly aid in dewatering alum sludge.
It is known to use H.sub.2 O.sub.2 in the waste water industry either as an oxidizing agent or as a means for treating filamentous bulking in activated sludge. U.S. Pat. No. 3,530,067 to Friedman discloses a method of treating sewage which employs H.sub.2 O.sub.2 to improve settling of biological materials. U.S. Pat. No. 4,221,661 to Shimizu et al. discloses a method of dehydrating organic sludge which employs high concentrations of H.sub.2 O.sub.2 for oxidizing the sludge. In Shimizu et al., the sewage sludge is acidified, heated and treated with H.sub.2 O.sub.2, and then is subsequently cooled down. U.S. Pat. No. 4,244,818 to Abson discloses a method for removing metallic impurities from sewage sludge which requires a pH adjustment of the sludge (by acidifying, metals go into solution) then treatment with H.sub.2 O.sub.2. Abson does not discuss an resulting improvement in dewatering and in fact is designed to have less metal retained in the sludge.
Several research reports and patents have discussed the benefits of treating sewage and waste water sludge with H.sub.2 O.sub.2. Some of the benefits include enhancing solids settling, retaining heavy metals in solution, reducing odors, reducing the amount of organic matter in the sludge, and enhancing the dewatering of the sludge by more or less conventional means. In all studies known by the applicant which are related to enhancing the dewatering of sewage sludge with H.sub.2 O.sub.2, very high concentrations of H.sub.2 O.sub.2 were utilized, i.e., in all studies more than 100 mg/l H.sub.2 O.sub.2 was used and the amount used typically ranged between 1000 mg/l to 2000 mg/1. In addition, in all cases other operations such as pH adjustment, polymer addition, heat treatment, etc., or some combination of the above, were an integral part of the process involving H.sub.2 O.sub.2 usage. These high concentrations of H.sub.2 O.sub.2, acid and alkali additions, and heating and cooling operations all singularly or in combination make the processes prohibitively expensive for augmenting conventional sewage sludge dewatering processes. Hence, no municipal sewage treatment plant in the United States of which the applicant is aware uses H.sub.2 O.sub.2 for enhancing dewatering of sewage sludge. Other conventional dewatering enhancement techniques have invariably proved more economical.
Sewage sludge, both raw and from biological sewage treatment processes, is quite different from alum sludge produced at a water treatment facility. The solid portion of the sewage sludge is primarily organic in nature. Conversely, a principal portion of the alum sludge is aluminum hydroxide or some variable thereof which is formed from the reaction of aluminum sulfate (which is added to the water) and alkalinity (either natural or, as is often the case, added). It is normal for the aluminum sulfate concentration added to the water being treated to exceed the concentration of suspended solids removed from the water. Thus, it is readily deduced and, in fact, has been shown that it is the aluminum hydroxide that is the principal constituent of alum sludge that is the root cause of dewatering problems with alum sludge. To date, there is no economical common dewatering enhancement process which is equally effective on sewage sludge and alum sludge and it is believed that this is because of a fundemental difference in the nature of the sludges.